It is current practice for cameras to use 3 CCD arrays to capture a colour picture. Each array senses the picture through a given colour filter. The 3 colours which are commonly used for this purpose are red (R), green (G), and blue (B).
Each CCD array outputs a video signal carrying information about the light received by each of its pixels. The 3 video signals are then generally digitised and put together as a digital stream.
The light received by such a camera from a given direction is thus defined by 3 colour values, for instance between 0 and 4,095 if coded on 12 bits. These data are then further processed to get a usable video stream. The further processing steps can take place either in the camera itself or in a studio.
One of these processing steps is applying a white balance. The goal of the white balance is to determine the weight of the various colours to get a picture with a desired look, generally as natural as possible. Typically white balance is performed by imaging a white reference surface which is illuminated with light of a specific colour temperature. The gain of each colour is adjusted until an image of the white reference surface is deemed to be white, i.e. non-coloured and substantially as viewed when illuminated with the specific colour temperature. It consists in applying a gain specific to the colour to each colour value.
As the range within which colour values out of CCD arrays vary is common to the 3 colours whereas the gains for white balance are generally different (depending on the colour), the range of colour values after white balance is specific to each colour.
Differently said, the maximum value (corresponding to saturation of the CCD array and generally called clipping level) after white balance is generally different for the various colours.
Due to this difference, which can be important (e.g. a gain of 14 for red, 5 for green and 8 for blue), a big part of the dynamic range (after white balance) from the colours with highest clipping level remains unused.